In communications networks, there may be a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the communications network is deployed.
For example, for future generations of mobile communications systems frequency bands at many different carrier frequencies could be needed. For example, low such frequency bands could be needed to achieve sufficient network coverage for terminal devices and higher frequency bands (e.g. at millimeter wavelengths (mmW), i.e. near and above 30 GHz) could be needed to reach required network capacity. In general terms, at high frequencies the propagation properties of the radio channel are more challenging and beamforming both at the network node at the network side and at the terminal devices at the user side might be required to reach a sufficient link budget.
In general terms, the use of beamforming could imply that the terminal devices will be not only operatively connected to the network node via a beam but also performs a handover between (narrow) beams instead of between network nodes of different cells. At higher frequency bands high-gain beamforming with narrow beams could be used due to more challenging radio propagation properties than at lower frequency bands. Each beam will only be optimal within a small area and the link budget outside the optimal beam will deteriorate quickly. Hence, frequent and fast beam switching is needed to maintain high performance. This is hereinafter referred to as beam management. One purpose of so-called beam management is thus for the network node to keep track of its served terminal devices with narrow beams (as used at the transmission and reception point (TRP) of the network node and/or the terminal devices) in order to increase coverage and throughput.
At higher frequency bands where diffraction and penetration losses might be comparatively high, there is a larger risk that served terminal devices will be blocked and lose connection to the TRP of the serving network node. Transmission diversity schemes might be used to mitigate this issue. Transmission diversity schemes are included in the Long Term Evolution (LTE) suite of telecommunications standards. On example of such transmission diversity scheme is space-time block coding based transmit diversity (or Space-Time Transmit Diversity; STTD for short). STTD utilizes space-time block codes (STBC) in order to exploit redundancy in multiple transmitted versions of a signal. STTD can be applied to single symbols in QAM code words or CDMA code words, or subcarrier symbols in OFDM based schemes.
However, using STTD with, say, four antenna ports, together with an analog antenna array, or panel, having two antenna ports in order to attain transmission diversity would require the TRP to be equipped with two analog antenna arrays (where two of the four antenna ports for the STTD transmission are connected to the two antenna ports per antenna array). Having two antenna arrays requires twice the cost as for one antenna array and requires twice the space as for one antenna array. Since antenna arrays could be expensive and space is a scarce resource in the TRP this option thus comes with some drawbacks.
Hence, there is still a need for improved mechanisms for transmission diversity.